THE AIR OR SWIMMINO BLADDER OF FISHES. 
385 
hence there seems to me to be no great difficulty in believing that 
natural selection has actually converted a swim-bladder into a 
lung or organ used exclusively for respiration.” * It surely is 
quite conceivable that under changed conditions, acting for a 
lengthened period, the Salamandroid Lepidosiren — which some 
naturalists of note still maintain to be more allied to amphibia 
than to fish — might gradually convert all its ichthycic characters 
into amphibian ones, just as I believ-e it has converted not only 
the swim-bladder and pneumatic duct into an air-breathing 
lung, trachea, and glottis, but also two pairs of the gills of the 
branchial arches into vascular channels, in order that it should 
be able to maintain a slow circulation, as a terrestrial animal, 
while encased in its cocoon of mud. I see, moreover, nothing 
improbable in the supposition that in the Lepidosiren we have a 
living witness of a fish in a transition stage towards becoming, 
under favourable conditions, a true amphibian ; and I can 
believe that amphibia are altered forms of fish, to which, in 
some cases, they bear a considerable resemblance; and I 
further think it not improbable that one of the steps in the 
transition — and a most interesting and important step it is — is 
made by the gradual conversion of the swim-bladder and 
pneumatic duct into a lung and trachea. Why some fish have 
a swim-bladder, and others of similar habits have none, can, I 
believe, be only accounted for by supposing, with Darwin, the 
absolute independence of new and old structures. So that cases 
may arise in which a swim-bladder and air-duct may become 
developed in fishes in which at present they are altogether 
absent ; for Kner has told us that these organs vary consider- 
ably even in fishes of the same species, and another naturalist 
has discovered a rudimentary swim-bladder in some of the 
Selachise. I am not aware that anything is known of the 
embryology and development of either species of Lepidosiren. 
A knowledge of the embryology would no doubt throw con- 
siderable light on the question of the position of this salaman- 
droid in the animal kingdom ; it would show us not only its 
relation to existing animals, but also its relations to extinct 
types. Its early embryology would probably reveal to us that 
at one period of its existence it has no trace of amphibian 
characters, that the cellular lung and trachea, with the sub- 
sequent persistent vascular canals, are not developed in the 
animal’s early stage. Let us hope that some enterprising 
naturalist will ere long succeed in acquiring a knowledge of 
the development of the Salamandroid Protopteri. 
* Origin of Species.” 
